A spheroidal graphite cast iron can be used as a structural material for realms where until now, heat-treated steels have been used exclusively, e.g. crankshafts for high-speed-Diesel engines, segmented shafts and pistons for hydraulic steering, trailer under-carriage kingpins for truck undercarriage couplings, driveshaft heads for motor vehicle drive shafts and coupling bodies for trailer couplings.
Among spheroidal graphite cast irons, types with a bainitic structure have been known and in use for a long time as materials with high tensile strength and high resistance to abrasion. For these materials, the cast bodies are heated to and held during the manufacture in general at austenitizing temperatures of 920.degree. C. to 950.degree. C. for 2 to 5 hours until the matrix carbon content has become more or less evenly distributed and any ledeburite, which might be present after casting, has been dissolved.
Reference may be had to U.S. Pat. No. 3,893,873, No. 3,273,998, No. 4,222,793, No. 3,549,431, and No. 3,860,457 in this regard.
After the austenitization, the castings are cooled down so rapidly that premature austenite transformation into pearlite, prior to reaching the isothermal bainite transformation temperature, is avoided. The casting is held at this temperature until the bainite reaction has been completed. Thereafter, as usual, it is cooled to room temperature (Giesserei 65 (1978) No. 4, pages 73 to 80).
There are also spheroidal graphite cast irons with a bainite structure, which derive from alloyed cast iron, where directly in connection with the cooling process following the casting process, a bainite reaction is produced because of the content of nickel, molybdenum, copper, manganese and possibly also chromium, depending upon the wall thickness of the casting (German patent document DE-OS No. 18, 08, 515).
A slightly alloyed spheroidal graphite cast iron with conventional quantities of carbon, silicon, phosphorus, sulphur and magnesium, a molybdenum content of 0.10 to 0.26% and a manganese content of 0.3 to 1.4% is also known from German patent document DE-OS No. 23, 34, 992. The casting which consists of the above is heat treated to an austenization temperature of 900.degree. C. and after being held at this temperature for a period of two hours, is cooled in a bath at 300.degree. C. in 10 minutes to 4 hours. As a result of these measures it is possible to obtain an austenite-bainite structure, which gives a tensile strength of 1100 N/mm.sup.2, an elongation to break of 10% and a hardness of 270 to 300 HB. (All percents are given by weight).